The electrochemical reduction of thioisonicotinamide in an aqueous medium

The electrochemical reduction of thioisonicotinamide NRCSNH2 has been examined in an aqueous medium between H-0 = - 1 and pH = 13.7. As shown by cyclic voltammetry and polarography, a global 2e(-) reversible transfer is followed by two successive first-order chemical reactions and a 2e(-), 2H(+) reduction (EC1C2E process). A mechanism is proposed, beginning with the 2e(-) reduction of NRCSNH2 to the formal diionized form NRC-(SH)(NH3+), the first chemical reaction, C-1, being a formal proton transfer which yields the 4-gemaminosulfanylmethylpyridine; the rate constant of this reaction is 2 x 10(4) s(-1) in acidic medium and diminishes in neutral medium. The second process, C-2 which is much slower, is actually made up of two reactions in parallel (with loss of NH3 or H2S), whose rate constants are in the ratio 4 to 1; the thioaldehyde and the imine which result from these reactions are easier to reduce than thioisonicotinamide. A comparison of the rates of deamination and dehydrosulfuration with those of dehydration and deamination previously obtained during the reduction of isonicotinic acid, isonicotinamide and their derivatives is made. (C) 1998 Elsevier Science Ltd. All rights reserved.